1. Field of the Invention
This invention relates to electric power-generating devices, such as wind turbines and ocean current turbines, and more particularly to a method and apparatus for controlling extendable rotors of wind or water turbines.
2. Description of the Prior Art
Rotating variable span airfoils have been in development for the past 20 years by the aerospace industry for use in helicopters and Vertical Take Off and Landing (VTOL) aircraft
Extendable rotor blades in rotating equipment, of which wind turbines are a subset, have been known in the art since the 1930""s (Cameron U.S. Pat. No. 2,163,482 and Ash U.S. Pat. No. 2,108,245). Numerous specific mechanical designs have been shown, such as the torque tube and spar assembly for a screw-driven extendable rotor blade (U.S. Pat. No. 5,636,969), the mounting arrangement for variable diameter rotor assemblies (U.S. Pat. No. 5,655,879), the variable diameter rotor blade actuation system using retention straps wound around a centrally actuated drum (U.S. Pat. Nos. 5,642,982 and 6,019,578), a locking mechanism and safety stop against over extension for a variable diameter rotor (U.S. Pat. No. 4,080,097), a variable diameter rotor with an offset twist (U.S. Pat. No. 5,253,979), a drive system for changing the diameter of a variable diameter rotor using right angle gears to interface with screw driven retraction mechanism (U.S. Pat. No. 5,299,912), as well as several others (U.S. Pat. Nos. 5,620,303; 6,030,177; 5,735,670; 5,655,879). In all cases, the prior art presents mechanisms for use as components of extendable rotor blade systems that either are parts of rotating aircraft equipment for helicopters or airplanes, or are described in more general terms as apparatus for use with any extendable rotor system.
U.S. Pat. No. 4,710,101, to Jamieson, granted Dec. 1, 1987, entitled xe2x80x9cWind Turbinexe2x80x9d discloses a conventional, horizontal axis, axial flow wind turbine for use in a wind turbine electrical generator set wherein an electrical generator is connected to the turbine for generation electrical energy.
Referring to FIG. 1 of Jamieson, the wind turbine comprises a conventional tower structure 10 that is stationary with reference to the wind flow. The tower has a rotatable head 11 upon which is mounted a rotor 12. The head is brought upwind by a vane 13 such that the rotor is in alignment with wind flow direction.
A movable nose portion 16 is located at or adjacent the leading edge of the blade and at or adjacent the tip of the blade. The nose portion is displaceable longitudinally of the blade, i.e. radially outwardly of the blade, from a normal retracted position. This moveable portion contributes to the lift of the arifoil section, and is moved to an advanced position in which drag is produced, to prevent unwanted increase in the speed of the rotation of the rotor.
U.S. Pat. No. 4,108,372, to Lippert, et. al, granted Dec. 25, 1979, entitled xe2x80x9cWind Rotor Automatic Air Brakexe2x80x9d also discloses a conventional, horizontal axis, axial flow wind turbine for use in a wind turbine electrical generator set wherein an electrical generator is connected to the turbine for generating electrical energy.
Referring to FIG. 1 of Lippert, et al, the wind turbine comprises a conventional tower structure 24 that is stationary with reference to the wind flow. The tower has a pivot arrangement 26 upon which is mounted a rotor 12.
An air brake 10 of the invention is mounted on the tips 12 of the blades 14 of the rotor 16. The rotor of the windmill is mounted on an output shaft 20, which is journaled in suitable bearings for rotation in the nose of a nacelle 22. Nacelle 22 is mounted on a tower 24 by means of the usual pivot arrangement 26, which allows the windmill to weathercock freely into the wind in alignment with wind flow direction. A suitable power train (not shown) converts the energy output of the rotor in a form suitable for utilization, such as an electrical generator connected for generating electrical energy.
Lippert, Jr. discloses a spring-loaded pivoting end plate braking mechanism 10 for a wind rotor. The end plate is hinged such that it is deployed by centrifugal force of a speed change detetced by a sensor whcih controls an actuator to effect the required positioning of the brake plate into the air stream. The break plate acts as an aerodynamic brake for wind turbines in over-speed conditions.
The prior art does not describe extendable rotor blade systems for wind or ocean current turbines combined with control of the loads they encounter.
The prior art shows rotor systems which operate within four regions: (1) at velocities below cut-in, (2) over a range of intermediate velocities which yield varying power production, (3) at higher velocities in which the turbines produce constant or slightly decreasing power in order to limit loads, and (4) at extremely high velocities in which the turbines cut-out. No prior art indicates operation within a fifth region in which rotor diameter is varied to maintain operation within a specified loads regime.
What is needed is a method of controlling wind or ocean current turbines in a way that increases energy production while constraining torque, thrust, or other loads below some level that is less than the loads that would be found if the rotor were allowed to produce peak system power while the rotors were fully extended, at all wind conditions from cut-in to cut-out wind speeds.
Briefly, the present invention relates to controlling an extendable rotor blade used in power generation equipment driven by slow moving fluids such as wind and water. The extendable rotor may consist of a number of general configurations. In one configuration, an airfoil with a span less than the outer radius of the turbine is controllably maneuvered outwards and inwards from the center of rotation along a load-bearing shaft, increasing and reducing the area swept by the airfoil during rotor revolution. In another configuration, the rotor consists of two main pieces: the main blade, and a blade extension.
As components of the turbine as a whole, these configurations present four major design variables: minimum rotor diameter (with the extension fully retracted), maximum rotor diameter (with the extension fully extended), the rated system power, and the rated system torque. Of slightly lesser interest, but of significance in isolated design cases as a limiting factor instead of the torque, are the rated system thrust (rotor drag) and blade root bending.
In accordance with an aspect of the invention, the mechanical torque (or thrust) delivered by the rotor is controlled such that the torque (or thrust) is limited to below a threshold value. An advantage of the invention is that it enables an extended rotor blade configuration to operate within adjustable torque and thrust load limits. This enables adaptation to a multitude of wind turbine powertrain manufacturers"" designs or to a variety of operating conditions through use of different control set points, and similarly enables retrofit of existing installed wind turbines.
Another advantage of the invention is that extendable blades offer the ability to enlarge or reduce the area swept by the blades, thereby increasing or decreasing the power capture for a given wind or ocean current velocity. Because the area swept by the rotor is proportional to the blades"" radius squared, small changes induced in the rotor radius (through extension or retraction of blade extensions) result in large changes in power capture. For example, a 25% increase in rotor radius results in a 56% increase in swept area. In addition, because wind or ocean currents may be intermittent, the turbines may operate for a significant portion of time in flows with velocities less than required to reach rated power output. A turbine capable of extending its swept area in low velocity periods could then significantly increase the energy generated during these times compared to a non-extendable rotor turbine, which also results in reduction of power output variability (intermittency).
An advantage of the present invention is that it provides an approach to turbine control that increases the value of power generating equipment with only limited additional cost.
The invention allows the rotor to yield significant increases in power capture through increase of its swept area, without any penalty in rotor torque or thrust loads delivered to the powertrain or the connected structures. This method is applicable to rotors either having blades that feather or having stall regulated airfoils.